An inverter is used to convert DC power into AC power. With the development of technology and the improvement of life quality, inverter especially multilevel inverter has been playing an increasingly important role. There are two kinds of inverters. They are multi-diode clamped inverter and flying capacitor inverter.
Four-level inverter produces four mutually different voltage levels. FIG. 1(a) shows the present multi-diode clamped four-level inverter topology unit and FIG. 1(b) shows the present flying capacitor four-level inverter topology unit. Their voltage utilization ratio is both one. They both need an additional circuit to increase the input voltage when higher output voltage is needed. It is noted that more than two capacitors are connected in series at the input side. Another additional circuit is needed to balance the voltage between these capacitors. These additional circuits reduce the system efficiency and increase the system cost.